The present invention relates to low power telecommunications such as cellular telephones. Specifically, a power amplifier is provided for efficiently amplifying radio frequency signals at multiple power levels.
The popularity of wireless telephones has grown significantly, with such systems offering multiple modes of operation. Wireless telephones configured to operate in a CDMA application provide power efficiencies which improve battery operation time by utilizing less transmit power when signal levels are adequate. In the CDMA type application full power is available up to 28 dbm (600 milliwatts) when it is necessary to communicate with the base station. However, in numerous situations where less power is sufficient, the wireless telephones operate in a back off mode, reducing the power output to 15 dbm (50 milliwatts) which will conserve battery life for the user while providing reliable communications for the user.
The back off mode can be implemented in several ways. One of the more common ways to implement back off mode operation reduces the radio frequency signal amplitude applied to the power amplification stage of the wireless telephone transmitter. This reduces the total power output, and provides some savings in battery power. However, in the backed off mode, the reduced drive to the power amplification stage results in the semiconductor power transistors having a power output efficiency reduced from 40% to as low as 2%. The corresponding efficiency loss for the power amplifier transistors burdens battery life, reducing the total power consumption efficiencies obtained by operating in the back off mode.
The wireless telephone transmitter must also deal with the problem of reflected power, as the antenna used for wireless telephones produces reflected power due to the positioning of the telephone during antenna operation. The reflected power interferes significantly with the linearity of the transmitter power amplification stages, as well as destroys the output power transistor in an extreme case. In the past, circulators have been used to redirect the reflected power directed to a load connected to a port of the circulator. Circulators tend to be lossy, however, further compounding the problem of efficiently producing radio frequency signals.
The present invention is directed to a power amplification system and method for producing low power radio frequency signals at multiple power output levels. The system and method in accordance with the present invention provides for the operation of power amplification stages at substantially their peak efficiency at multiple power output levels.
The signal to be amplified by the power amplification system is divided into first and second signals. The first and second signals are applied to first and second driver stages where they are amplified to a first level. When the system is to be operated at the higher output power level, the first and second driver stage output signals are further amplified with first and second output amplifiers operating at peak efficiency. The signals from the first and second output amplifiers are recombined in a directional coupler to provide a transmit signal for an antenna. When the system is to be operated in a backed off, low power mode, they are directly connected to the directional coupler for providing the output signal. In either the high power or low power mode of operation, all amplifier stages operate at their peak efficiency and reflected power is diverted to an output port of the coupler, thereby protecting the power amplification system from reflected power.
In a preferred embodiment of the invention, the divided signals are produced by a directional coupler producing first and second quadrature signals. The coupler for recombining the output signals of the first and second output amplifiers or the first or second driver stages, depending on the mode of power operation, is also a directional coupler. By using the directional coupler, it is possible to efficiently and inexpensively couple the signal to an antenna load without suffering effects of reflected power from the antenna load.